With technological advancement and demand for mobile instruments, demand for secondary batteries as an energy source is rapidly increasing.
In particular, as interest in environmental problems is increased, a great deal of research on electric vehicles and hybrid electric vehicles has been conducted in order to replace conventional automobiles using fossil fuels such as a gas-oil vehicle, a diesel vehicle, etc., which are a major cause of air pollution. Such an electric vehicle or hybrid electric vehicle is generally driven using a nickel-metal hydride battery type power source, however, a lithium secondary battery with high energy density and discharge voltage is now actively being studied and partially commercialized in the related art.
Although an anode active material for a lithium secondary battery generally includes a carbon material, lithium metals or sulfur compounds may also be used. As to cathode active materials for a lithium secondary battery, lithium containing cobalt oxide (LiCoO2) is widely used. Additionally, other lithium transition metal oxides, for example, lithium containing manganese oxides such as LiMnO2 with a lamellar crystal structure, LiMn2O4 with a spinel crystal structure, etc., lithium containing nickel oxide (LiNiO2), and the like have also been proposed.
However, the lithium transition metal oxide used for a cathode active material has drawbacks such as low electrical conductivity, low ionic conductivity due to use of a non-aqueous electrolyte, in turn not satisfying high charge-discharge rate properties, and so forth.
In order to solve such problems, some conventional techniques such as coating of a surface of a cathode active material or surface treatment of the same have been proposed. For example, a coating method of a cathode active material with conductive polymer which includes applying a conductive material to the cathode active material, in order to decrease contact resistance at an interface between the cathode active material and an electrolyte or side product generated at a high temperature, has been disclosed. However, improved cathode active materials with sufficient cell characteristics still need to be developed.
Furthermore, high energy density means possible exposure to risks and risks such as ignition, explosion, etc. may become more serious as the energy density is increased.
Accordingly, in spite of extensive research and studies into different approaches, satisfactory results have not yet been attained. Because of increase in energy density in proportion with increasing complexity and multi-functionality of mobile instruments, safety of the same is more significant and rate properties of a lithium secondary battery for EVs, HEVs, power tools, etc. should be further improved.
However, since safety and rate properties have substantially contradictory tendencies, it is very difficult to simultaneously enhance both the foregoing characteristics and very little research and/or discussions regarding the same are currently being conducted.